Photogrammetry is the science of obtaining accurate information about an object by recording and measuring photographic images of the object. Included within the definition of photogrammetry is the process of making surveys and maps through the use of photographs. Normally, aerial photography is employed for this purpose thus making it possible to survey and map large areas with a minimum of time and effort. Photogrammetry is particularly useful in the mapping of remote areas of limited accessibility.
The camera equipment utilized for photogrammetric purposes must be of rugged construction and must be relatively simple to operate. The images being photographed are normally at infinity, so that depth of field and focusing are not a primary problem in photogrammetric and aerial photography. Since any distortion in the image will introduce errors, photogrammetric cameras, which are normally box cameras, are equipped with very expensive lenses for maximum accuracy. As mentioned, the camera housing, mounting devices and the like must be relatively rugged in order to withstand the rigors of aerial photography and surveying.
Conventional view cameras comprise a housing which is normally rectangularly shaped and which has a rear face adapted for receiving a film holder. The front wall of the camera housing may be hinged for movement between a closed position with the lens received in a protected position in the camera housing and an open extended position with lens exposed. The lens is mounted by a lens board which is disposed within the housing and the inner surface of the front wall of the camera housing is provided with a track over which the lens board moves when the front wall is in the open position. A light tight seal between the lens board and the interior of the housing is provided by a collapsible bellows or similar device. Suitable drive means are provided for moving the lens board along the track to focus the lens. The shutter, shutter speed control mechanism, and aperture control mechanism may be built into the lens body or may be separate from the lens, as is well known in the art, and a shutter control mechanism such as a sheathed plunger mechanism extends through the housing for operating the shutter. For aerial photography, the lens focus, aperture and shutter speed are normally preset so that all that is necessary in the air is to change the film and operate the shutter control.
As previously mentioned, accuracy is of the highest importance in photogrammetric work and great care must be taken to factor in any inaccuracies due to the film and the lens. For this reason, very expensive and highly accurate lenses, usually certified for accuracy by the U. S. Bureau of Standards or other qualified agency, are employed in photogrammetric cameras. Processing of the exposed film is done with great care, even to the point of transferring the negative from film to a positive on glass plates in order to avoid any further inaccuracy which might be caused if the negative were transferred to plastic sheet or paper materials such as is the case with ordinary photographs.
In addition to errors induced by the lens and the film, another source of inaccuracy in photogrammetric work, which cannot be compensated for or factored in, is due to the camera body itself. Commercially available camera bodies can produce inaccuracies due to the many dimensional tolerances which are inherent in the design and manufacture of the camera body and which would be of no consequence in normal photography. In addition, inaccuracies are generated when a conventional camera body is exposed to variations in temperature, such as may be encountered in aerial photography. For example, if the lens position is set in the camera at room temperature, such as is normal practice, at low temperatures sufficient dimensional changes in the camera body will occur to induce errors in the photographs which cannot be compensated for in normal practice. Yet another cause of inaccuracy is due to lens buffeting when the photographs are taken from aircraft.
To overcome the aforementioned problems, camera bodies designed especially for photogrammetric purposes are available. Such camera bodies are constructed of heavy, materials which can withstand the handling, wind buffeting and the other factors encountered with aerial photography. Such camera bodies are, however, useful for a single purpose only, are heavy and are very expensive.